This option comes in several different variants (see below).
None of the variants invokes the underlying compiler; they all provide
information on how the underlying compiler would have been invoked had
--showme not been used. The basic --showme option outputs the command line
that would be executed to compile the program. NOTE: If a non-filename argument
is passed on the command line, the -showme option will not display any additional
flags. For example, both "mpicxx --showme" and "mpicxx --showme my_source.c"
will show all the wrapper-supplied flags. But "mpicxx --showme -v" will only
show the underlying compiler name and "-v".

--showme:compile

Output the compiler
flags that would have been supplied to the C++ compiler.

--showme:link

Output
the linker flags that would have been supplied to the C++ compiler.

--showme:command

Outputs the underlying C++ compiler command (which may be one or more tokens).

--showme:incdirs

Outputs a space-delimited (but otherwise undecorated) list
of directories that the wrapper compiler would have provided to the underlying
C++ compiler to indicate where relevant header files are located.

--showme:libdirs

Outputs a space-delimited (but otherwise undecorated) list of directories
that the wrapper compiler would have provided to the underlying linker
to indicate where relevant libraries are located.

--showme:libs

Outputs a
space-delimited (but otherwise undecorated) list of library names that the
wrapper compiler would have used to link an application. For example: "mpi
open-rte open-pal util".

--showme:version

Outputs the version number of Open
MPI.

See the man page for your underlying C++ compiler for other options
that can be passed through mpicxx.

Conceptually, the role of
these commands is quite simple: transparently add relevant compiler and
linker flags to the user’s command line that are necessary to compile /
link Open MPI programs, and then invoke the underlying compiler to actually
perform the command.

As such, these commands are frequently referred to
as "wrapper" compilers because they do not actually compile or link applications
themselves; they only add in command line flags and invoke the back-end
compiler.

Open MPI is comprised of three software layers: OPAL
(Open Portable Access Layer), ORTE (Open Run-Time Environment), and OMPI
(Open MPI). There are wrapper compilers for each layer; each layer’s wrapper
only links in the libraries relevant for that layer. Specifically, each
layer provides the following wrapper compilers:

OPAL

opalcc and opalc++

ORTE

ortecc and ortec++

OMPI

mpicc, mpic++, mpicxx, mpiCC (only on systems
with case-senstive file systems), mpif77, and mpif90. Note that mpic++,
mpicxx, and mpiCC all invoke the same underlying C++ compiler with the
same options. All are provided as compatibility with other MPI implementations.

The Fortran wrapper compilers for MPI (mpif77 and mpif90) will be inoperative
and will return an error on use if Fortran 77 / Fortran 90 support was
not built into the MPI layer.

mpicxx is a convenience wrappers
for the underlying C++ compiler. Translation of an Open MPI program requires
the linkage of the Open MPI-specific libraries which may not reside in one
of the standard search directories of ld(1). It also often requires the
inclusion of header files what may also not be found in a standard location.

mpicxx passes its arguments to the underlying C++ compiler along with
the -I, -L and -l options required by Open MPI programs.

The Open MPI Team
strongly encourages using the wrapper compilers instead of attempting to
link to the Open MPI libraries manually. This allows the specific implementation
of Open MPI to change without forcing changes to linker directives in users’
Makefiles. Indeed, the specific set of flags and libraries used by the
wrapper compilers depends on how Open MPI was configured and built; the
values can change between different installations of the same version of
Open MPI.

Indeed, since the wrappers are simply thin shells on top of an
underlying compiler, there are very, very few compelling reasons not to
use mpicxx. When it is not possible to use the wrappers directly, the -showme:compile
and -showme:link options should be used to determine what flags the wrappers
would have used. For example:

It is possible to make the wrapper
compilers multi-lib aware. That is, the libraries and includes specified
may differ based on the compiler flags specified (for example, with the
GNU compilers on Linux, a different library path may be used if -m32 is
seen versus -m64 being seen). This is not the default behavior in a standard
build, but can be activated (for example, in a binary package providing
both 32 and 64 bit support). More information can be found at:

The string that the wrapper compilers insert into the command line
before invoking the underlying compiler are stored in a text file created
by Open MPI and installed to $pkgdata/mpicxx-wrapper-data.txt, where $pkgdata
is typically $prefix/share/openmpi, and $prefix is the top installation
directory of Open MPI.

It is rarely necessary to edit this file, but it
can be examined to gain insight into what flags the wrappers are placing
on the command line.

By default, the wrappers use
the compilers that were selected when Open MPI was configured. These compilers
were either found automatically by Open MPI’s "configure" script, or were
selected by the user in the CC, CXX, F77, and/or FC environment variables
before "configure" was invoked. Additionally, other arguments specific
to the compiler may have been selected by configure.

These values can be
selectively overridden by either editing the text files containing this
configuration information (see the FILES section), or by setting selected
environment variables of the form "OMPI_value".